As is known, a radial tire comprises a tread, two inextensible beads, two sidewalls connecting the beads to the tread, a carcass reinforcement and a belt placed circumferentially between the carcass reinforcement and the tread. The belt is made up of various rubber plies (or “layers”) which may or may not be reinforced by reinforcing elements such as cabled threads or monofilaments, of the metal or textile type.
The belt generally consists of several superposed belt plies, sometimes called “working” plies or “crossed” plies, the generally metallic reinforcing cables of which are placed so as to be practically parallel to one another within a ply but at an angle from one ply to another, that is to say they are inclined, whether symmetrically or not, to the median circumferential plane. These crossed plies are generally accompanied by various other auxiliary rubber plies or layers, which vary in width depending on the case and may or may not comprise metal reinforcing elements. Mention may in particular be made of what are called “protective” plies responsible for protecting the rest of the belt from external attack and especially from perforations, or else what are called “hooping” plies having metallic or non-metallic reinforcing elements oriented substantially along the circumferential direction, (so-called “zero-degree” plies), irrespective of whether they are radially outside or inside with respect to the crossed plies.
As is known, such a tire belt must meet various, often contradictory, requirements, in particular:
it must be as rigid as possible at low deformation, as it contributes substantially to stiffening the tire crown;
it must have as low a hysteresis as possible, in order, on the one hand, to minimize heating of the inner region of the crown during travel and, on the other hand, to reduce the rolling resistance of the tire, this being synonymous with fuel economy; and
finally, it must have a high endurance, in particular with respect to the phenomenon of separation, i.e. cracking of the ends of the crossed plies in the shoulder region of the tire, known as “cleavage”, which in particular requires metal cables that reinforce the belt plies to have a high compressive fatigue strength, even in a relatively corrosive atmosphere.
The requirement of high endurance is particularly important in the case of tires for industrial vehicles, such as heavy goods vehicles or civil engineering machinery, which are designed in particular to be able to be retreaded one or more times when their treads reach a critical stage of wear after prolonged travel or usage.
Furthermore, the tire belts for industrial vehicles comprise, as is known, one or more “protective” crown plies or layers located beneath the tread and covering the working crown ply or plies, which are designed to protect the rest of the belt from external attack, tearing or other perforations.
These protective plies or layers must be sufficiently flexible and deformable so that, on the one hand, they best follow the shape of the obstacle on which the belt bears during travel and, on the other hand, they prevent the penetration of foreign bodies radially thereinto. As is known, to meet such criteria requires the use, in these protective layers, of reinforcing elements or cables having a high elasticity and a high fracture energy.
For the reinforcement of the protective crown plies, the hooping plies and the working crown plies of tires for heavy industrial vehicles, it is general practice at the present time to use multistrand cables, especially multistrand cables having a single layer (i.e. with no central core of one or more other strands) of K×(L+M) construction, the K elementary strands of which are assembled, wound simultaneously in a helix as a single layer, with an assembly pitch PK. Each of the K two-layer elementary strands, of L+M construction, itself comprises a plurality of steel wires also wound together in a helix as two concentric layers (an inner layer of L wires and an outer layer of M wires).
Such multistrand cables having a K×(L+M) construction, especially of the type having a high elongation, are well known and have been described in many patent documents, in particular for reinforcing protective crown plies of tires for industrial vehicles such as heavy goods or civil engineering vehicles (see for example EP 1000074, U.S. Pat. No. 6,475,636, U.S. Pat. No. 7,458,200, WO 2004/003287 or US 2005/0183808, WO 2004/033789 or U.S. Pat. No. 7,089,726 or else RD (Research Disclosure) No 33877, June 1992, 488-492.
As is well known to those skilled in the art, these multistrand cables must be impregnated as much as possible by the rubber in the tire belts that they reinforce so that said rubber penetrates as much as possible into the spaces between the wires constituting the strands. If this penetration is insufficient, empty channels then remain along the strands, and corrosive agents, for example water, capable of penetrating the tires, for example as a result of the tire belt being cut or otherwise attacked, travel along these channels through said belt. The presence of this moisture plays an important role, causing corrosion and accelerating the fatigue process (so-called “fatigue-corrosion” effect) compared to use in a dry atmosphere.
All these fatigue effects, generally grouped together under the generic term “fatigue-fretting corrosion”, are the cause of progressive degeneration of the mechanical properties of the cables and strands and may, under the most severe running conditions, affect the lifetime of the tires.